Part of T68930
Now two other mirror options that can be enabled simultaneously: Mirror Y and Z.
Reviewers: campbellbarton
Reviewed By: campbellbarton
Subscribers: ThatAsherGuy
Differential Revision: https://developer.blender.org/D5720
Part of T68836
`transform conversions.c` is a file that is getting too big (almost 10,000 lines).
So it's a good idea to split it into smaller files.
differential revision: https://developer.blender.org/D5677